Privacy system



Aug. 13, 1946. D. MITCHELL 2,405,599

PRIVACY SYSTEM Filed Sept. 24, 1941 2 Sheets-Sheet l NJ mw! T,

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PRIVACY SYSTEM Filed Sept. 24, 1941 2 Sheets-Sheet 2 Patented Aug. 13, 1946 PRIVACY SYSTEM Deren Mitchell, Bound Brook, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application September 24, 1941, Serial No. 412,070

7 Claims.

This invention relates to a transmission system and, particularly, to a method of and means for assuring secrecy in the transmission of messages.

An object of the invention is the assurance of secrecy in the transmission of messages and the prevention of intelligibie reception of the message by unauthorized listeners.

Applicant has disclosed in an earlier patent (2,129,860, issued September 13, 1938), a privacy arrangement in accordance with which previously recorded noise or spurious speech is applied to the transmission channel during the silent intervals, i. e., when the regular talkers are not speaking, whereby thereis produced on the channel a practically continuous train of sound composed of the useful speech and the noise or spurious speech. This train of sound is in the nature of a meaningless jargon, the useful speech being so blended with the noise or spurious speech that it will be practically impossible for unauthorized listeners to interpret the message.

The arrangement contemplated by the present invention is of the same general type as that disclosed by the above patent, but is an improvement over that prior arrangement in respects the speech frequency band. As a result, signal frequencies outside the speech band, which were required in the instance of the prior arrangement referred to, are not necessary in the present arrangement.

In accordance with the general method of operation contemplated by the present invention the same frequency band is occupied by the spurious speech and the useful speech. The frequency spectrums of the two are caused to diier in a small but distinctive manner, however, and this difference is utilized at the receiving end of the channel to control operation of the necessary switching devices.

A full understanding cf the operation of the arrangement contemplated by the present int vention as well as appreciation of the various advantageous features thereof may be gained from consideration of the following detailed description in connection with the accompanying drawings, in which:

Fig. 1 illustrates schematically a two-Way, four-wire radioI telephone system embodying features of the present invention; and

Fig. 2 illustrates in greater detail one terminal of the system of Fig. 1.

The diagram of Fig. 1 is not an actual circuit diagram but rather a single line layout, each line indicating a transmission path, In this layout a normal break in a path is indicated by separated arrowheads and a normal make by connecting arrowheads. In order to indicate that a transmission element is to be controlled by a device, an arrow is 'shown pointing from the device towards the element. An arrow directed to a make point indicates that the path will be disabled at that point by operation of the control device andan arrow directed at a break point indicates that the break in the path will be eliminated, i. e., the path will be enabled, by operation of the control device.

Referring now to Fig. 1, the arrangement contemplated by the invention is shown incorporated in a two-way, four-wire circuit forming a part of a radio telephone system. The two terminals, West and East, of the system are represented, the circuit at the West terminal comprising a one-way transmission circuit EA for connecting the output of West subscribers station II to the West-East radio transmitter I2 and a one-way transmission circuit WB for connecting the East-West radio receiver I3 to the input of West subscribers station I I. Similarly, at the East terminal, a one-way transmission circuit EB connects the West-East radio receiver I4 to the input of East subscribers station I5, while .the output of subscribers station I5 is connected to the East-West radio transmitter I6.

The West terminal transmitting circuit EA is normally disabled (i. e., when subscriber W is not speaking) so far as the output of subscribers station Il is concerned at control point I1 by suitable means, for example, a short circuit, controlled by switching relays operating, in turn, under control of amplier-detector 2. At this time, however, spurious speech from source 22, a phonograph for example, is applied to circuit EA and transmitted thereover to radio transmitter I 2; radio transmitter I2 transmits this spurious speech to the East terminal.

An unauthorized person listening-in at this time on 4the transmission from the West terminal, therefore, would hear only the spurious speech emanating from source 22. In order to make things even more confusing to an unauthorized listener, the spurious speech may be recorded by the person ordinarily talking from subscribers station I l in order that the spurious speech and the useful speech may be in the same voice."

rThe frequency band occupied by the useful speech and that occupied by the spurious speech are the same; this frequency band will be referred to as fl-fio.

Band elimination filter 23 is included in the path over which the spurious speech from source 22 is applied to circuit EA; this filter eliminates from the frequency band of the spurious speech a predetermined, relatively narrow, portion. The eliminated portion of the band will be referred to as f2-fs.

In order that subscriber E, the authorized speaker at subscribers station l5, may not be subjected to the spurious speech which is now beingtransmitte'd as described above, circuit EB is held open at point 2d by suitable means, for example a short circuit, applied and removed under control of dierential relay 25. Relay 25 is provided with two windingswinding 2@ energization of which tends to operate the armature up and winding 2l energization of which tends to voperate the armature down When the armature is in the down position, as illustrated, circuit EB isr open at point 2li but when the armature is operated to the up position, circuit EB is closed through or enabled at point 24; these operations will be described in detail in connection with Fig.V 2. 1

Y windings 2:6 and El are connected Vto circuit EB through respective lines il! and e2, amplifierdetector #l being connected inline l 'and' ampliier-detector lli being connected in line e2. Bandpass filter l5 is also included in line lil; this filter is designed to pass only a small band of frequencies including the portion fz-fs of the spurious speech band which was eliminated by band elimination filter 23;

Y During the time spurious speech from source 22 is being received at the East terminal, the armature of relay 2E will be held in the down position as winding 2l will be energized by current rectified by amplier-detector 134. Little or no energy will be supplied to winding 2.6 by amplipoint Il? thereby removing the spurious speechV from the circuit.

The useful speech originating at subscribers station il passes, therefore, over circuit EA to radio transmitter l2, is radiated therefrom and is picked up by radio receiver ld. A small portion fe-fc of the useful speech band is eliminated by band elimination lter 5i in order to heighten the confusion between the useful speech and spurious speech and thereby increase the privacy. The eliminated portions ft-'s and'fz-fs, while differing so far as their relative positions in the frequency spectrum fi--fm are concerned, are of equal width.

A portion of the received useful speech energy passes through line e2 to amplifier-detector 44 and after rectification is applied to winding 21 of relay ze. Portion f2-f3 of the useful speech band passes through iilter 45 to amplifier-detec- 4 tor 43 and after rectification is applied to winding 25 of relay 25.

Amplier-detectors t3 and 44 are so designed that the sensitivity of the first mentioned is appreciably greater than that of the second so that even though the energy of the portion fz-fa is considerably less than that of the total band (which is applied to amplifier-detector 44) the rectified energy applied to relay winding 26 will be substantially greatei-V than that applied to winding 21. Relay 25 will be operated to the up position, therefore, thereby enabling circuit EB at point 2li for transmission of the useful speech to subscriber E at subscribers station l5. A portion of the speech energy is by-passed through amplier-detector 52Aand causes, through operation of a relay or other suitable means, disabling of circuit WA atvpoint 53. This prevents undesirable operation of the transmitting controls of theEast terminal by echoes.

When the regular talker at subscribers station H ceases speaking, the circuits immediately return to the condition illustrated and spurious speech is again applied to the transmission channel. An unauthorized listener, therefore, upon gaining access to the channel would hear a practically continuous jargon comprised partly of useful speech partly of spurious speech. Interpretation or" the message by recognization of the legitimate portions and mental masking of the spurious portions would be very dicult thereby rendering the message highly secret so far as unauthorized listeners are concerned. Confusion and frustration on the part of unauthorized listeners and persons attempting to crack the privacy system are heightened by the fact that both the useful speech band and the spurious speech bandv have similar, small portions eliminated therefrom and may be increased still further if the spurious speech and the useful speech be in the same voice.

Referring now to Fig. 2 a more detailed showing is provided of the West terminal of Fig. l. Subscribers station i I is coupled to transmitting line 'il and receiving line I2 by transformer 13, network 'M Vproviding a proper balance in the well-known manner. SpeechY energy emanating 'from subscribers station I! passes through arnpliiier 'I5 after which it follows two parallel paths, one path leading through amplifier-detector 2l and the other path leading through band elimination lter 5| and windings of transformer T! to radio transmitter l2.

Phonograph turntable Si is provided together with suitable driving mechanism 32, record 83 being rotatably supported on the turntable. A pick-up device E34 is operatively associated with record 83 and serves to translate the mechanical undulations on the record into corresponding electrical undulatio-ns for transmission over line Record 83 preferably has recorded thereon spurious speech and this spurious speech may, as pointed out above, have been recorded in the voice of the subscriber who ordinarily talks from station Il. The frequency band occupied by the spurious speech is the same as that' occupied by the speech energy which emanates from subscribers station Il; for convenience this band is designated as fi-fio. The output of pick-up 84 is connected to windings of transformer 11 by line 4 6, .band elimination lter 23 being included in this line. Filter 2S is so designed that it extracts orneliminates from the frequency band fi--fio a definite small portion which we will designate 2-3.

It might be pointed out here that band elimination filter 5|, which is included in line 1| and through which the speech energy emanating from subscribers station passes, is designed to eliminate from the frequency band fl-flu occupied by that energy a small portion of the band which is of the same width as that removed by filter 23 but which occupies a different relative position in the frequency spectrum; we shall designate the portion eliminated by filter 5| as f5-fs Radio receiver I3 is associated with receiving line 12 which, in turn, -is inductively coupled to subscribers station by transformer 13, amplifier 85 being connected in line 12. Two paths for speech energy arerprovided in shunt to line 12, one path leading through amplifier-detector 86 to winding 81 of differential relay i0! and the other path leading through band-pass filter |02 and amplifier-detector |03 to winding |04 of the differential relay. Band-pass lter |02 is designed to pass only a small band of frequencies including that portion of the frequency band i--fio which is removed by band elimination filter 23, that is, the portion designated fz-fs.

A third path for speech energy provided in shunt to receiving line 12 passes through amplier-detector |05 to relay |06.

The condition illustrated in Fig. 2 is that prevailing when the regular talker at subscribers station is not speaking. Transmitting line 1| is disabled so far as the output of station is concerned by the short-circuiting path completed through the armature and break contact of relay |01. Spurious speech from record B3 is picked up by unit 80 and applied over line liS to windings of transformer 11. It will be recalled that the spurious speech occupies the band fi-fm and that the portion fz-fs is removed from the energy supplied to transformer 11 by band elimination filter 23. The spurious speech, minus the portion ier-fa, is, therefore, supplied to radio transmitter l2 by which it is transmitted to the distant receiving station. The arrangement of the windings of transformer 11 and network |08 associated therewith is such that this energy is not diverted through the short circuit applied to line 1| at the break contact of relay |01.

The receiving circuits of the distant terminal, which are not illustrated, are similar to the receiving circuits of the West terminal illustrated in Fig. 2 and it is believed that the effect of the currents transmitted by radio transmitter E2 upon the distant terminal circuits may be better visualized if we transfer our attention for the moment to the receiving circuits of Fig. 2 and assume that spurious speech (fi-fio minus the portion f2-f3) is being received from the radio transmitter of the distant terminal (corresponding to radio transmitter i2).

A portion of the energy received by radio receiver I3 and applied to receiving line 12 is diverted through amplifier-detector 8G and after rectification thereby is applied to winding 81 of differential relay lili. Substantially no energy is diverted through amplifier-detector |03 at this time as band-pass lter |02 is designed to pass only a small band of frequencies including the frequency portion fz--fs and it will be remembered that this portion is missing from the spurious speech band being received. Armature of relay |0| is operated to the up position, therefore, by energization of winding 81 and the energizing path of relay ||2 is open. Receiving line 12 is, therefore, short-circuited at this time by a 6 path including the armature and break contact of relay ||2 and transmission of the spurious speech being received to subscribers station is prevented, and the subscriber, therefore, is subjected to no annoyance.

Resistance pad H3 provided in receiving line 12 is arranged to prevent the short circuit applied to the line at relay I2 from interfering with the transmission of received energy into the paths leading to amplifier-detectors 8&3 and |03.

Turning our attention again now to the transmitting circuits of Fig. 2 and assuming that the subscriber at station starts talking, a portion of the speech energy applied to transmitting line 1| is diverted through amplifier-detector 2| and, after rectification thereby, causes operation of relays |61 and Illi.

Operation of relay |01 removes the short circuit from line 1| and enables the line for transmission of the speech energy. Operation of relay H4 applies a short circuit to line $6 thereby disabling the line for transmission of the spurious speech. The spurious speech is, therefore, removed from the transmission channel and the useful speech being produced at subscribers station is applied thereto.

It Will be remembered that the useful speech occupies the same frequency band (f1-fm) as that occupied by the spurious speech. A relatively small portion, f5-fs, of the useful speech frequency band is removed by band elimination filter 5|, this removed portion being of the same magnitude as the portion removed from the spurious speech band; the relative positions of the two removed portions in the respective frequency spectrums are different however. The arrangement of the winding of transformer 11 is such that transmission of the useful speech therethrough to radio transmitter l2' is not 1nterfered with by the short circuit applied to line i6 by operation of relay IG.

At this time, therefore, the useful speech produced at subscribers station (minus the portion fs-fs) is transmitted over line 1| to radio transmitter |2 and transmitted thereby to the distant terminal. In order to better visualize the operation of the receiving circuits of the distant terminal which are not illustrated in Fig. 2, let us again consider the receiving circuits of the West terminal illustrated in Fig. 2 and assume that useful speech energy is now being received from the distant terminal and that the spurious speech is not being transmitted.

As before, a portion of the energy supplied to receiving line 12 by radio receiver I3 is diverted through amplifier-detector 8S and, after rectification thereby, energizes winding 81 of differential relay lili. At this time, however, energy is also diverted through amplifier-detector |03 as band-pass filter |02 passes a small band of frequencies including the portion ,f2- f3 of the frequency band and it will be recalled that this portion, while removed from the spurious speech band is present in the band of the useful speech transmitted. This latter energy, after rectication by amplifier-detector |03, energizes Winding Wfl of differential relay |l|. Amplifier-detector |03 is designed for a substantially higher sensitivity than is amplifier-detector 86 so that, while the energy applied thereto may be less than that applied to amplifierdetector 8B, the rectified energy applied to winding |04 willr still be appreciably greater than that applied to winding 81. Armature is, therefore, operated to its lower or make position at thistime.

Operation of armature Hl to the lower position completes an operating path for relay H2 traced from battery H5, make contact of relay HEI, winding of relay H2 to ground HG. Operation of relay I l2 removes the short circuit from receiving line 'i2 and enables the line for transmission of the useful speech to subscribers station li. A portion of the energy is diverted through amplifier-detector 195 and, after rectiflcation thereby, causes operation of relay i536 which operation is effective to short-circuit transmitting line 'il and prevent undesirable operation of the Vtransmitting controls by echoes.

Again considering now the transmitting circuits of Fig. 2, as soon as the subscriber at station ll ceases talking, the circuits return to the condition illustrated and spurious speech is again` applied to the channel. In accordance with the general features of, operation, therefore, the regular authorized subscribers hear only the useful speech and are not subjected to the spurious speech. Unauthorized persons listening in on the channel, however, hear a meaningless, substantially continuous jargon Vcomprising snatches of useful speech, interspaced during what-would otherwise be the silent periods, with the kspurious speech. Confusion is heightened if the spurious speech be recorded in theY voiceY of the subscriber who ordinarily talks from station il while cracking is rendered much more difficult Vby the fact that blank or no-energy portion are to be found in both the useful and the spurious speech bands due to the action of the band eliminationv filters.

If desired, the pass bandsof the different filters may be changed from time to time thereby,.in effect, varying the combinaticnY of the system and it will be understood that other types of sharply tuned circuits may be used instead of the filters. It will be understood, further, that the arrangement may be operated in conjunction with other types of privacy devices now well known, for example the arrangements described respectively by R. W. Chestnut et al. Patent 1,829,783 and Patent 1,981,114 Vof applicant and another.

While certain specific embodiments of the invention have been selected for detailedv description, the invention is not, of course, limited in its application to the embodiments described. These embodiments should be taken as illustrative of the invention and not as restrictive thereof. Y

What is claimed is:

l. In a speech transmission system, a source of useful speech Waves, a source of spurious speech waves, said useful speech waves and said spurious speech Waves as produced by their respective sources occupying the same frequency band, a transmissionV channel, means for applying spurious speech waves from said source to said transmission channel, means for applying useful. speech from said source to said transmission channel, means for removing a certain definite portion of the frequency band from the speech energy applied to said channel from one of said sources, means operating in response to application of said useful speech energy to said channel to remove said spurious speech from said channel, receiver means and means at a receiving point of the system selectively operated by wave components having frequencies in said certain definite portion of said frequency band 8 for ,controlling the operativeness of said receiver means.

2. In a speech transmission system, a source of useful speech waves, a source of `spurious speech waves, said useful speech Waves and said spurious speech waves as produced by their respective sources occupying the same frequency band, a transmission channel, means for applying spurious speech waves from said source to said transmission channel, means for removing a certain definite portion of the frequency band from'the spurious speech energy applied to said channel, means for applying useful speech from said source to said transmission channel, means for removing a certain definite portion of the frequency band from the useful speech energy applied to said channel, means operating in response to application of said useful speech energy to said channel to remove said spurious speech from said channel, receiver means and means at a receiving point of the system selectively operated by Wave components having frequencies in said denite portion removed from the frequency band of the spurious speech for controlling the operativeness of said receiver means, the portion of the frequency band removed from the spurious speech energy and the portion of the frequency band removed from the useful speech energy being of equal widths but occupying different relative positions in the respective frequency spectrums.

3. In a speech transmission system, including a West terminal and an East terminal, a source of useful speech waves -at said West terminal, a source of spurious speech waves at said West terminal, said spurious speech waves and said useful speech waves as produced by said sources occupying the same definite frequency band, a transmission channel between said terminals, a line at said West terminal for applying spurious speech waves from said last-mentioned source to said channel, means included in said line for excluding a certain definite portion of the frequency band occupied by said spurious speech waves, means at said East terminal for disabling said transmission channel at a point at said East terminal, and means operating in response to application of said excluded portion of said frequency band to enable said channel at said point.

4. In a speech transmission system including a West terminalr and an East terminal, a source of useful speech Waves at said West terminal, a source of spurious speech waves at said West terminal, said useful speech waves and said spurious'spc-aech waves as produced by their respective sources being of confusingly similar sounds, a transmission channel between said terminals, means for# applying useful speech waves from said source to said channel, and means for applying spurious speech waves from said Vsource to said channel, means for removing acertain definite portion of the frequency band from the spurious speech energy applied to said channel whereby the frequency spectrum of the spurious speech waves transmitted over said channel is distinguishable from the frequency spectrum of the useful speech waves transmitted thereover, and switching means at the East terminal operating to'distinguish between the useful speech waves and the spurious speech waves on the Ybasis of the difference between the respective frequency spectrums.

5. In a speech privacy system of the type whichtincludes a' transmitting terminal, areceiving terminal, a channel between said terminals and means at the transmitting terminal for impressing spurious speech on the channel during each pause in the useful speech, the method of discriminating between the spurious speech and the useful speech at the receiving terminal which includes producing a distinguishable difference between the frequency spectrum of the useful speech and the frequency spectrum of the spurious speech at the transmitting terminal and utilizing this difference to control the operation of switching means at the receiving terminal.

6. In a speech transmission system including a West terminal and an East terminal, and having a telephone transmitter, a radio transmitter and a line for connecting said telephone transmitter and said radio transmitter at said West termina1 and a radio receiver, a telephone receiver and a line for connecting said radio receiver and said telephone receiver at said East terminal, means for disabling said East terminal line at a definite point therein, a source of spurious speech at said West terminal, the spurious speech Waves as produced by said source occupying the same frequency band as that occupied by useful speech Waves as produced by said telephone transmitter, means for applying useful speech Waves produced by said telephone transmitter to said West terminal line, a second line at said West terminal for applying spurious speech waves produced by said source to said rst West terminal line, a wave lter in said second line for eliminating a denite, relatively small portion from the frequency band of the spurious speech Waves applied thereover to said rst West terminal line, and switching means at said East terminal operating only in response to application to said East terminal line of the portion of the frequency band eliminated by said lter to enable said East terminal line at said previously mentioned point.

7. In a speech transmission system including a West terminal and an East terminal, and having a telephone transmitter, a radio transmitter and a line for connecting said telephone transmitter and said radio transmitter at said West terminal and a radio receiver, a telephone receiver and a line for connecting said radio receiver and said telephone receiver at said East terminal, means for disabling said East terminal line at a definite point therein, a source of spurious speech at said West terminal, the spurious speech waves as produced by said source occupying the same frequency band as that occupied by useful speech'waves as produced by said telephone transmitter, means for applying useful speech waves produced by said telephone transmitter to said West terminal line, a second line at said West terminal for applying spurious speech waves produced by said source to said rst West terminal line, a Wave filter in said second line for eliminating a definite, relatively small portion from the frequency band of the spurious speech waves applied thereover to said rst West terminal line, a two-position switching device at said East terminal, said device operating to a rst position in response to application to said East terminal line of the spurious speech waves after removal of said portion by said filter and operating to a second postion in response to application to said East terminal line of the useful speech waves, and means operating in response to operation of said device to said second position for enabling said East terminal line at said previously mentioned point.

DOREN MITCHELL. 

